[raster] Support reprojection on import

Description

shp2pgsql recently got a switch to reproject vectors during import (-s <from>:<to>). It would be nice to have the same switch in raster2pgsql, especially for handling overviews that are more annoying to deal with after import.

One problem with having reprojection done by GDAL is that we cannot use a "srid" but rather we need an "srs". Constructing it from a query into spatial_ref_sys would break current isolation between the loader and the database.